The present invention relates to a compression molding machine, in which a work piece is clamped and compressed between dies together with resin so as to mold the work piece with the resin.
Many types of machines have been developed to manufacture semiconductor devices. Especially, transfer molding machines have been used to mold semiconductor chips with resin. In the transfer molding machine, resin is supplied from pots to cavities, which are formed in an upper die and/or a lower die, via runners and gates, so as to mold work pieces set in the cavities.
In the transfer molding machine, for example, a plungers of a multi-plunger unit are respectively provided in pots of the lower die. The plungers are supported by a pressure equalizing unit and driven by a transfer mechanism. After resin tablets are supplied into the pots and the work pieces are set in the cavities of the lower die, an open-close mechanism clamps the work pieces between the dies. Then, the transfer mechanism drives the plungers to exert pressure on the resin melt in the pots, so that the resin is supplied into the cavities (see Japanese Patent Gazettes No. 5-285977 and No. 9-155910).
The movable die of the resin molding machine should have enough opening stroke so as to set the work pieces in the cavities and take out molded products therefrom. The dies should generate a required clamping force. The movable die should be moved at a required clamping speed so as to secure proper manufacturing efficiency.
The open-close mechanism has a pair of toggle links, which act as reduction units for reducing rotational speed of an electric motor and transmitting torque therefrom. In each of the toggle links, link pieces having different length are pivotably connected. The toggle link moves at a relatively high speed, with lower reduction ratio, at a die-opening position; the toggle link moves at a relatively low speed, with higher reduction ratio, at a die-closing position (see Japanese Patent Gazettes No. 5-285977 and No. 9-155910).
In another type of the open-close mechanism, a ball bearing screw is connected to a movable platen. The ball bearing screw is turned to open and close dies (see Japanese Patent Gazette No. 5-84766).
Compression molding machines have been also used to mold work pieces. In the compression molding machines, resin of a required amount is supplied to work pieces, then the work pieces are clamped and compressed between a fixed die and a movable die together with the resin. By compressing the work pieces together with the resin, the work pieces can be molded with the resin. No resin paths, e.g., runners, gates, are formed in the dies, so maintenance of the compression molding machine is easy. Further, the compression molding machine is preferably used to mold thin work pieces having thickness of, for example, about 0.8 mm. Lead frame type packages, e.g., QFN (Quad Flat Non-leaded), SON (Small Outline Non-leaded), and CSP (Chip Size Package), in which semiconductor chips are matrically arranged on one face of a substrate or a circuit board, are preferably molded by the compression molding machine. Note that, in the QFN or SON package, preferably a tape is adhered on one side face of a lead frame.
In the compression molding machine, a movable die is moved at a lower speed when the work pieces are actually clamped and released; the movable die is moved at a relatively high speed in other states as well as the transfer molding machine (see Japanese Patent Gazette No. 4-14419).
In the case of molding the thin work pieces, e.g., QFN, SON, CSP, by the transfer molding machine, there are some disadvantages.
The transfer mechanism connected to the plungers is located in a lower part of the lower die, so through-holes, in which the plungers are moved upward and downward, are formed in a lower platen, which holds the lower die. With this structure, parts of the lower platen corresponding to the through-holes apply no pressure to the dies when the dies are closed. Namely, pressure applied to the dies is partially different, so that the dies are slightly deformed. By the slight deformation of the dies, flashes are formed in molded products.
If the toggle links are used in the open-close mechanism, length of link pieces and positions of pivoting points are sometimes slightly deviated within ranges of manufacturing allowances, so that the lower platen is move with slight inclination. Therefore, degree of parallel between the movable platen and the fixed platen is made lower, so that pressure applied to the dies is partially different and the dies are slightly deformed. By the slight deformation of the dies, flashes are formed in molded products and quality of the molded products are made lower.
In the case of driving the movable die by the ball bearing screw, the movable platen can be maintained parallel to the fixed die while opening and closing the dies. But, a high power motor is required to control the clamping speed and generate the clamping torque as well as the toggle links, so that the molding machine must be large. It takes a long time to move the movable platen with an ordinary stroke, e.g., 200–300 mm, so productivity is low.
If the work pieces are set and the molded products are taken out in a limited space between the opened dies, working efficiency is low. And, the space is too narrow to install a dispenser for supplying the liquid resin.
In the case of molding the work pieces having thin package sections by the transfer molding machine, amount of the resin is small and a conveying distance of the resin is long, so there is possibility of solidifying the resin while conveying. Therefore, it is difficult to uniformly supply the resin to package sections of the work pieces, so the conveying distance of the resin should be short.
On the other hand, in the case of molding the work pieces by the compression molding machine, if the movable die is moved at a fixed speed, a flowing speed of the resin immediately after beginning the clamping action is not equal to that immediately before terminating the clamping action. If semiconductor chips are wire-bonded to a substrate, the flowing speed of the resin must be low so as not to damage bonding wires; the clamping speed must be the minimum speed, so that it takes a long time to close the dies and productivity must be lower. For example, the clamping speed of the movable die should be reduced to 1/10– 1/100 of the conventional machine. Thus, the compression molding should be executed with considering the relationship between the clamping speed and the flowing speed of the resin so as to maintain quality and productivity without damaging products.